1. Field of the Invention
The present invention relates to a reversible thermosensitive recording material capable of recording and erasing images by utilizing its property that the transparency changes reversibly between a transparent state and an opaque state depending upon the temperature thereof.
2. Discussion of Background
Hitherto, various reversible thermosensitive recording materials have been proposed, and, among them, the most typical one is a thermosensitive recording material comprising a thermosensitive recording layer which contains a matrix resin such as a polyvinyl chloride resin, and an organic low-molecular-weight material such as a higher fatty acid dispersed therein as disclosed in Japanese Laid-Open Patent Applications 54-119377 and 55-154198.
In the reversible thermosensitive recording material of this type, the property of the thermosensitive recording layer that its transparency reversibly changes depending on the temperature is utilized for recording and erasing images in the recording layer. However, the state of the thermosensitive recording layer is changed from opaque to transparent in a narrow temperature range of 2.degree. to 4.degree. C. It is therefore difficult to control the temperature at which a partially opaque recording material is entirely changed to transparent, or an entirely opaque recording material is made partially transparent to form transparent images therein.
In order to eliminate the above shortcoming in the prior art, the inventors of the present invention have proposed a reversible thermosensitive recording material comprising a thermosensitive recording layer which contains a specific low-molecular-weight material and a material which easily constitutes a eutectic mixture with the low-molecular-weight material as disclosed in Japanese Laid-Open Patent Applications 63-39378 and 63-130380. The state of this thermosensitive recording layer changes from opaque to transparent in a temperature range wider than the above-mentioned range. However, the temperature range has been extended to a relatively low temperature region, so that images formed in the recording material occasionally disappear when the recording material is preserved at temperatures between 50.degree. and 60.degree. C.
Under these circumstances, the inventors of the present invention further found that the temperature range in which the state of the thermosensitive layer is changed from opaque to transparent can be extended to a high temperature region by incorporating thereinto an aliphatic dicarboxylic acid having a high melting point as disclosed in Japanese Laid-Open Patent Application 2-1363.
Thus, the temperature range can be successfully extended, and at the same time the preservability of the recorded images can also be improved. However, in the case where a protective layer is provided on the thermosensitive recording layer, the transparency of the thermosensitive recording material is deteriorated, possibly due to exudation of the aliphatic dicarboxylic acid contained in the recording layer to the protective layer.